Cable calculation

[Sponge bob big pants]

Hi All, 

I am very rusty on cable calculations as I haven’t really needed to do it very often and kind of need a bit of reassurance. 
 

I need to supply a 3 phase sub board, 45 meter run in a duct from one switch room to another.

main supply is 100Amp 

Sub board is a 3 phase 100Amp main switch 

I was thinking I would need 35mm SWA 4 core with 20mm separate earth. Does that sound correct. 
 

Any help is appreciated.

 

[kerching]

20mm earth? Good luck with that one

far too little info provided

how do you get to 35mm?

just curious

 

[Sponge bob big pants]

I used a cable calculator online. What would you suggest then?? 

 

[kerching]

What is Zdb?

will Ze at far end be low enough to lift the OCP on a fault?

ambient temp?

is duct surface or buried?

mind you a contractor on site has just run a 50m 4c 10 mm SWA S brain with a 63A MCB and he thinks it's ok 

 

[Sponge bob big pants]

To be honest I don’t know the Ze

it is in a duct under ground with 5 other SWA 

Ambient temp 30

10mm sounds like he is penny pinching! 
 

 

[Badger]

63 amps I'd have said. Minimum 16mm 4 core possibly 25mm 

 

[Sponge bob big pants]

Hi m8,

it’s 100Amp main switch

a sub contractor installed a 16mm SWA for this already but it has been flagged that it is too small. 
 

they now want the cable calculation. This is what I am struggling with. 
 

cheers

 

[Badger]

Hi m8,

it’s 100Amp main switch

a sub contractor installed a 16mm SWA for this already but it has been flagged that it is too small. 
 

they now want the cable calculation. This is what I am struggling with. 
 

cheers

I was replying to kerching 

I would have said 16mm would not have been big enough h to start with from. Day 1 as I'd have said min 25mm ideally 35mm due to possible. Load at 100 Amps I would have to do a calculation but I'm out at the moment 

 

[Sponge bob big pants]

ok sorry bud

 

[kerching]

63 amps I'd have said. Minimum 16mm 4 core possibly 25mm 

My thoughts exactly...can't get him to understand that he has used thermosetting ratings BUT his terminals are not 90° so he needs to derate to PVC ratings....anyway, not my monkeys, not my circus

 

[Sponge bob big pants]

I was replying to kerching 

I would have said 16mm would not have been big enough h to start with from. Day 1 as I'd have said min 25mm ideally 35mm due to possible. Load at 100 Amps I would have to do a calculation but I'm out at the moment 

It’s the calculation part I’m not confident with. I would think a 35mm 4 core would be fine. If you can help me out with the calculation when you have time that would be great.

 

[Badger]

My thoughts exactly...can't get him to understand that he has used thermosetting ratings BUT his terminals are not 90° so he needs to derate to PVC ratings....anyway, not my monkeys, not my circus

I wouldn't have put 63 amps onto a 10mm in any case I'd have upped it from a safety sizing POV to be on the safe side at that level of current and protection.

 

[Sponge bob big pants]

Hi Badger

I was replying to kerching 

I would have said 16mm would not have been big enough h to start with from. Day 1 as I'd have said min 25mm ideally 35mm due to possible. Load at 100 Amps I would have to do a calculation but I'm out at the moment 

Would you have any time to help me out with this calculation?? Like I said I don’t have the Ze but just an idea would be helpful. Cheers

 

[Sidney]

Load?

Earthing arrangement?

PFC?

Ze/Ra?

Distribution or final circuit?

Any lighting?

Installation method?

 

[Sponge bob big pants]

Load?

Earthing arrangement?

PFC?

Ze/Ra?

Distribution or final circuit?

Any lighting?

Installation method?

The load is 100Amps

just now the board is on a TT 

but I can run an earth in with the new SWA 

Ze 0.08

its in a duct buried 

45 meter run

all that we are putting onto this board is 6 car charging points 

6 x 32Amps

 

[Sponge bob big pants]

The load is 100Amps

just now the board is on a TT 

but I can run an earth in with the new SWA 

Ze 0.08

its in a duct buried 

45 meter run

all that we are putting onto this board is 6 car charging points 

6 x 32Amps

Sorry it’s 4 chargers

4 x 32amps

 

[Andy™]

http://jarsoftelectrical.co.uk/

 

[Sidney]

Using 50mm CSA 70c SWA I get VD of 3.6 using the resistive load column.

R1+R2 of 0.034 ohms giving Ze of 0.114.

PFC 4.95Ka.

Adiabatic has 1.06mm for copper and 2.40mm for steel armour for a 5 second disconnection time.

 

[Sponge bob big pants]

Thanks for that Sidney, it’s appreciated.

I got to say I didn’t think it would be a 50mm SWA. Don’t know if I would get that through the duct

 

[Sidney]

35mm is only good for 98 amps buried direct for three phase.